1. Field of the Invention
The present invention relates to a multi-color beam exposure system for exposing photosensitive material to light beams of multiple colors.
2. Background Art
FIG. 1 shows a drum scanner 10 used in conventional color printers and the like. There is provided in the drum scanner 10 three light emitting diodes (LED) 13C, 13M, and 13Y for respectively emitting three beams 13CB, 13MB, and 13YB of different colors (for example, an infrared-ray (IR) beam having a wavelength of 805 nm, a red (R) beam having a wavelength of 665 nm, and a yellow (Y) beam having a wavelength of 570 nm, respectively). There is also provided in the drum scanner three focusing optical systems 13CL, 13ML, and 13YL situated so that the respective beams 13CB, 13MB, and 13YB are focused to form focused images 13CI, 13MI, and 13YI on a color photosensitive material 12 wound around a drum 11. The focused images act to develop three color C (cyanogen), M (magenta), and Y (yellow) on the photosensitive material.
In a known exposure system for the drum scanner 10 a single line is exposed to the focused beams during each revolution of the drum 11 (in the direction of an arrow a in the drawing). The drum 11 is stopped after each revolution and the focused images 13CI, 13MI, and 13YI are displaced a predetermined distance in the axial direction b of the drum 11. The foregoing series of operations are repeated until the focused beams have scanned all the scan lines. This process is called intermittent sub-scanning feeding. FIG. 2 shows scanning lines 14 on the color photosensitive material 12 when exposure is carried out by the foregoing intermittent sub-scanning feeding. As seen from FIG. 2, the scanning lines 14 are obtained on the photosensitive material 12 in the form of parallel straight lines disposed at predetermined regular intervals.
An alternative known exposure system, uses an exposure process known as continuous sub-scanning feeding. In this exposure process the revolution of the drum 11 (in the direction of the arrow a) is synchronized with the displacement of the focused images 13CI, 13MI, and 13YI in the axial direction b of the drum 11. FIG. 3 shows scanning lines 15 on the color photosensitive material 12 when exposure is carried out by the continuous sub-scanning feeding method. In this case, the scanning lines 15 are obtained on the photosensitive material 12 in the form of parallel oblique lines disposed at predetermined regular intervals. Since the scanning lines 15 trace a spiral on the drum 11, it is necessary to dispose the focused images 13CI, 13MI and 13YI offset from each other so that the focused images 13CI, 13MI, and 13YI are aligned along each scanning line 15, as shown in FIG. 3.
Both systems suffer the drawback of requiring a long exposure time because in each exposure of only one scanning line can be performed during each revolution of the drum.
Although the foregoing drawback can somewhat ameliorated by increasing the drum speed, there is a practical limit on drum speed. Another limit on drum speed is the required exposure time necessary to effect development. As the light output from LED's is limited as is the sensitivity of the color photosensitive method there is a minimum exposure time necessary to properly expose the photosensitive material.
Further, since in the conVentional system the light emitting diodes 13C, 13M, and 13Y for emitting color beams and the focusing optical systems 13CL, ML, and 13YL are mounted on a sub-scanning exposure head 4 as shown in FIG. 4, there is the additional problem that the sub-scanning exposure head 4 is so heavy its maximum speed is severely limited because of its large inertia force. It is also low in positioning accuracy because of large friction drag, particularly in the case where high-speed intermittent driving is performed by a drum scanner 10. Still further, the sub-scanning exposure head 4 is very bulky thus restricting reduction of drum scanner 10 size.